Is the Focus on “Molecules” Obsolete?

AC06-FrontMatter ARI 12 May 2013 13:24 by Ursinus College on 08/28/13. For personal use only. Annual Review of Analytical Chemistry 2013.6:1-29. Downloaded from www.annualreviews.org AC06CH01-Whitesides ARI 13 May 2013 10:37 Is the Focus on “Molecules” Obsolete? George M. Whitesides Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138; email: [email protected] Annu. Rev. Anal. Chem. 2013. 6:1–29 Keywords The Annual Review of Analytical Chemistry is online analysis, teaching, simplicity, soft lithography, paper diagnostics, at anchem.annualreviews.org self-assembly This article’s doi: 10.1146/annurev-anchem-062012-092633 Abstract by Ursinus College on 08/28/13. For personal use only. Copyright c 2013 by Annual Reviews. The technologies developed in analytical chemistry have defined in spec- All rights reserved tacular detail the properties of molecules. The field now faces enormously important and interesting problems of which molecules are only a part: for example, understanding the nature of life; helping to manage megacities, oceans, and atmospheres; and making health care (especially diagnostics) Annual Review of Analytical Chemistry 2013.6:1-29. Downloaded from www.annualreviews.org affordable and relevant. The emergence of these problems involving molecular systems raises the issue of how (and what) analytical chemistry should teach. Historically, it has been essential to chemistry in teaching the science of measurement. As complicated analytical techniques proliferate, it must consider how to balance teaching the uses of sophisticated devices and the fundamentals of analysis and measurement. This review (by an admiring but nonanalytical chemist) sketches the essential role of analytical methods— especially simple ones made up on the spot—in guiding research in new fields, with examples from self-assembled monolayers, soft lithography, paper diagnostics, and self-assembly; and suggests issues in teaching. 1 AC06CH01-Whitesides ARI 13 May 2013 10:37 1. INTRODUCTION Question and answer: “What does analytical chemistry do?” “It determines the composition and structure of matter, and particularly of molecules.” Correct answer ...or so yesterday? Much of science follows the “scientific method.” This ponderous but reliable process is a wheel: postulation of theory, formulation of a hypothesis testing the theory, experimental test of the hypothesis, comparison of experimental results with theory, modification of theory, and so on. Measurement (or “analysis,” which arguably includes both measurement and interpretation) is the axle around which this wheel turns. So, everything in science is—in this sense—“analytical.” Almost everything in science requires some form of “analysis.” In chemistry, the specialty that focuses on analysis of matter, and especially of molecules (and the specialty that also develops new methods of measurement), is “analytical chemistry.” Analytical chemistry sometimes leads chemical science (by enabling new types of experiments); sometimes it follows (by adding useful precision to previously developed capability, or by making methods more user friendly). Regardless, it is essential to the chemical enterprise. Chemistry defines itself as the science of atoms, molecules, and matter; it tends to focus on molecules. It has become very adept within that definition: Chemists can make, or obtain from nature, an enormous range of molecules and determine their structures, even at extraordinary levels of complexity, with sensitivities now routinely approaching those required to examine single molecules and atoms. This type of work, with its molecular focus, has been the core of the field and will (and must) certainly continue. At the same time, analytical chemistry is blurring around its disciplinary edges. It is increasingly difficult to tell what is “chemistry” and what is “biology” or “materials science” or “cognitive science,” or “health care,” or “energy.” Further, the role that society expects the sciences, and especially chemistry, to play is changing: from developing focused technical methods to solving (or helping to solve) large-scale problems, both in areas of direct, immediate public concern (health care, robots and jobs, environmental monitoring) and in areas of fundamental understanding (“What is life?” “How does climate instability develop?” “How does the human brain think?”). With this broad sweep of styles of problems—from sequencing proteins to helping to manage megacities—what will “analytical chemistry” become? Will it be a discipline that focuses on a small part of a bigger problem—one in which molecules may be only a paragraph in a much longer story—or will it lead the integration of measurement, analysis, and interpretation of information across a number of disciplines? by Ursinus College on 08/28/13. For personal use only. I am not an analytical chemist, and am not competent to answer this broad question; and in any event, I do not know the answer. I am, however, a constant user of analytical methods, someone who often thinks in terms of analysis, a developer of new analytical methods when I need them, and a scientist who is wholly dependent on measurement. And, whether I “know” or not, I have opinions. Why did I think it might be worth anyone’s time to read an essay that I—a nonexpert—might Annual Review of Analytical Chemistry 2013.6:1-29. Downloaded from www.annualreviews.org write on analytical chemistry? I have two reasons. First, I have been using analytical methods for 60 years, and the changes in the field in that time have been large, interesting, and instructive for the light they shed on the continuing evolution of chemistry; perhaps the perspective is useful. Second, it seems to me that analytical chemistry is facing some substantial choices, and sometimes “change” is more obvious from outside a field than inside. In this review, I intend to make five points: 1. Analytical chemistry is extremely important—probably even more important than analytical chemists think. Everything in science requires measurement; analytical chemists are experts in the science of measurement, not just in determining the structures of molecules and the compositions of mixtures of molecules. 2 Whitesides AC06CH01-Whitesides ARI 13 May 2013 10:37 2. In new fields—fields in which substantial expertise in analysis does not already exist—simple analytical methods often have far greater value than do more precise and sophisticated ones. The rate of progress in a new field often depends as much on the ability to fail rapidly—to understand in the morning why a hypothesis is incorrect, and to try another one in the afternoon—as on precision. Developing simple analytical methods (and teaching students how to develop such methods) is an important and sometimes critical part of exploratory research. 3. “Good enough” is often good enough, even in a developed field. Academic scientists tend to favor complexity and “world-class performance.” For users, the balance between convenience, cost, and performance may strongly favor convenience or cost over performance, and it can be as challenging and interesting to develop a truly workable, simple system—one that can be used by others—as it is to develop a highly specialized system that will probably seldom (or never) be replicated. 4. The most important problems for analytical chemistry may be changing from “molecules” to “functions.” 5. For all of the value of its technology and methods, and for all the importance of the information that it provides, one of the most important contributions of analytical chemistry may be in teaching new scientists and engineers how to make measurements. As the field has broadened, and as more and more science, instrumental design, and software accumulate in commercial instrumentation, understanding how to balance teaching the fundamentals of measurement with the training needed to operate very sophisticated instrumentation, without letting “analytical chemistry” drift into “the care and feeding of black boxes,” is an important challenge, and one that is now without a clear solution. 2. WHAT DOES CHEMICAL ANALYSIS DO? (OR, WHAT DOES “ANALYTICAL CHEMISTRY” SEEM TO BE, TO AN OUTSIDER?) 2.1. Early Impressions Stick Interestingly (at least to me), my first experiences in laboratory were in analytical chemistry. Even before high school, I worked during the summers as the lowest-level technician in the small by Ursinus College on 08/28/13. For personal use only. company owned by my father. He made a number of products, including (especially at that time) various formulations of coal-tar black and short-fiber chrysotile asbestos designed to fill, for example, the expansion joints between slabs of concrete in roads and runways. My job was to determine pour-point viscosities for coal-tar blacks. These measurements involved taking a sample of hard, black pitch; putting it in a clean copper cup with a small hole in the bottom; heating the cup and its pitch to a temperature specified in an ASTM publication; and then carefully measuring the Annual Review of Analytical Chemistry 2013.6:1-29. Downloaded from www.annualreviews.org time required for a specified volume of the melted pitch to drip out of the hole into a volumetric cylinder. Afterward, I cleaned the apparatus again to an immaculate state, using a mixture of benzene and carbon tetrachloride (I quite liked the smell of the mixture). The combination—a mixture of chemicals (polycyclic aromatics, short-fiber chrysotile asbestos, benzene, carbon

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